The present invention relates in general to lithography used in the manufacture of semiconductor devices, and particularly to an alignment system.
The manufacture of semiconductor devices uses a lithographic technique. It is often necessary to align or locate an object with a known reference coordinate system. In microcircuit lithography, the image of a reticle is projected onto a photosensitive substrate or wafer which is reproduced forming circuit patterns. Different processing steps often require different layers to be sequentially formed on the substrate to form a semiconductor device. Accordingly, it is often necessary to relocate the substrate with a high degree of accuracy. The substrate must be accurately positioned relative to prior patterns formed thereon. Alignment accuracy is often critical and typically is required to be less than 0.1 micron. Generally, an alignment mark or target is placed on a substrate to be aligned and is located with reference to a second object such as an alignment sensor. The position of the alignment mark, and therefore the substrate, is determined with respect to the alignment sensor coordinates. This is a critical step in alignment. The position of an alignment mark is normally defined with respect to the wafer plane and the center of symmetry of some portion of the alignment mark pattern. Typically an alignment sensor has an alignment axis that is normal to the plane of the substrate or wafer. Generally, alignment sensors require the use of a special alignment mark pattern, or a relatively restricted class of alignment mark patterns, that is placed on a substrate or wafer.
An alignment system is disclosed in U.S. Pat. No. 5,477,057 entitled xe2x80x9cOff Axis Alignment System For Scanning Photolithographyxe2x80x9d issuing to David Angeley et al on Dec. 19, 1995, which is herein incorporated by reference. Therein disclosed is an alignment system having multiple detectors for detection of light scattered and reflected from alignment marks placed on a wafer. Another alignment system is disclosed in U.S. Pat. No. 5,559,601 entitled xe2x80x9cMask And Wafer Diffraction Grating Alignment System Wherein The Diffracted Light Beams Return Substantially Along An Incident Anglexe2x80x9d issuing to Gallatin et al on Sep. 24, 1996, which is herein incorporated by reference. Therein disclosed is a grating-grating interferometric wafer alignment system utilizing a coherent illumination for determining alignment from the return electromagnetic radiation intensity. Another alignment system is disclosed in U.S. Pat. No. 5,767,523 entitled xe2x80x9cMultiple Detector Alignment System For Photolithographyxe2x80x9d issuing to McCullough on Jun. 16, 1998, which is herein incorporated by reference. Therein disclosed is a detector having a photosensitive surface covered with an opaque layer having predetermined openings therein combined with a matched reticle having predetermined apertures.
While these and other alignment systems have performed adequately for their intended purpose, there is an increasing need for improving the accuracy of alignment systems. Additionally, there is a need for an alignment system that can utilize a relatively large class of alignment mark patterns.
The present invention is an alignment sensor that is self referencing to the center of symmetry of any mark having one hundred and eighty degree symmetry. An alignment mark having one hundred and eighty degree symmetry placed on the substrate is illuminated with coherent illumination. The image of the alignment mark is divided into two images. The images are rotated one hundred and eighty degree with respect to each other, and are coherently recombined with an interferometer. With proper phasing of the interferometer paths the recombined image will interfere constructively or destructively in amplitude in a detectable and identifiable way or combine to form detectable and identifiable polarization states when the axis of the alignment sensor is located at the center of the alignment mark symmetry. The alignment mark may be any class of mark having 1800 symmetry, such as alternating spaces and lines, a grid pattern, or a checkerboard pattern, including other symmetrical patterns. Preferably, a prism is used to form the two images, rotate the two images with respect to each other, and interferometrically recombine the two images. A detector detects the interference as a result of the recombined two images and locates the center of the alignment mark to accurately detect the position of the substrate.
Accordingly, it is an object of the present invention to provide improved alignment in a lithographic system used in the manufacture of semiconductor devices.
It is an advantage of the present invention that a mark having one hundred and eighty degree symmetry is used.
It is a feature of the present invention that two images of the alignment mark are coherently recombined in an interferometer to accurately identify the center of the alignment mark.
It is a feature of the present invention that a prism is used to divide, rotate, and recombine the image of an alignment mark having one hundred and eighty degree symmetry.
These and other objects, advantages, and features will become readily apparent in view of the following detailed description.